1. Field of the Invention
The present invention relates to a crystallized glass and in particular, to a crystallized glass for information magnetic recording medium substrate. In particular, in substrates for magnetic recording medium to be used in various information magnetic recording units, especially perpendicular magnetic recording media, media for patterned media, discrete track media, etc., the invention provides a crystallized glass having a reduced content of an alkali metal for the purpose of suppressing a lowering of magnetic properties of a recording medium or reducing a defect to be caused due to alkali corrosion, having low viscosity properties adaptive with press molding on a mass production level, having an extremely smooth substrate surface adaptive with a low flying height of a magnetic head, having high Young's modulus and low specific gravity properties and having excellent mechanical properties.
The “information recording medium” as referred to in the invention means an information magnetic recording medium which can be used in, for example, a fixed hard disk, a removal hard disk or a card type hard disk which is used as a hard disk of a personal computer; a hard disk for digital video camera, digital camera or audio; a hard disk for satellite navigation system, a hard disk for cellular phone; or a hard disk for various electron devices.
2. Description of the Related Art
In recent years, for the purposes of conforming with an multimedia application of a personal computer and dealing with large data such as motion videos and voices by a digital video camera or a digital camera, an information magnetic recording unit with large capacity becomes necessary. As a result, in the information magnetic recording medium, in order to increase the areal recording density, there is a tendency that the bit or track density is increased and that the size of a bit cell is reduced. For that reason, a magnetic head must be worked more closely to the disk surface than that which has been adopted so far.
Furthermore, when the recording density exceeds 100 Gb/in2, such a small magnetic domain becomes thermally instable. Therefore, an in-plane recording system reaches a physical limit against a demand of high recording density exceeding 100 Gb/in2.
In order to adapt with this matter, adaptation and mass production of a perpendicular magnetic recording system are being advanced. In this perpendicular magnetic recording system, since the axis of easy magnetization is a perpendicular direction, it is possible to make the bit size extremely small. Also, since the medium has a desired film thickness (from 5 to 10 times that in the in-plane recording system), a reduction of demagnetizing field and an effect due to shape magnetic anisotropy can be desired. Therefore, problems to be caused in high density of the existing magnetic recording system in an in-plane direction, including a reduction of recording energy and thermal instability, can be solved, and as compared with the in-plane magnetic recording system, a remarkable enhancement in recording density can be realized. In view of these matters, in the perpendicular magnetic recording system, it has already become possible on a mass production level to attain a recording density of 100 Gb/in2 or more on a practical use level, and studies regarding the recording density exceeding 300 Gb/in2 are already performed.
In this perpendicular magnetic recording system, since the magnetization is performed in a perpendicular direction to the medium surface, different from existing media having an axis of easy magnetization in the in-plane direction, a medium having an axis of easy magnetization in the perpendicular direction is used. As the recording layer of the perpendicular magnetic recording system which is studied and put into practical use, there are exemplified various alloy films, for example, a barium ferrite film, a Co-γFe2O3 or Co based alloy, an Fe based alloy such as FePt, an Ni based alloy, etc.
Now, in such a magnetic recording medium, in order to achieve refinement and formation in a perpendicular direction of a magnetic crystal particle, it is required to increase the fabrication temperature. Furthermore, according to recent studies, there is the case where annealing is carried out at a high temperature (from about 500 to 900° C.) for the purpose of enhancing the magnetic properties.
In order to adapt with this matter, the substrate is required such that the material shape hardly changes before and after the annealing treatment and that it is excellent in heat resistance. Also, following the high density, in order to make it possible to attain a low flying height of a magnetic head, an extremely smooth surface smoothness is considered to be necessary.
In addition, in order to make it possible to continuously use a magnetic recording medium over a long period of time, basic problems to be caused due to the elution of an alkali from the substrate, such as (a) a lowering in magnetic properties of the recording medium (the alkali component diffuses into the recording medium, thereby lowering magnetic properties of the medium) and (b) deposition of a defect onto the substrate surface (the eluted alkali component diffuses into the surface of the recording medium to become a compound, which is then formed as a foreign matter on the surface), must be solved.
The former phenomenon is described in more detail. First of all, with respect to the alkali elution phenomenon, in case of manufacturing a crystallized glass which requires an alkali component as a constitutional component of a crystal phase, the concentration of the alkali component of a raw glass must be made higher than the stoichiometric amount necessary in the crystal phase, and after the crystallization, the alkali component which has not been consumed remains in the glass matrix phase. According to this, the matters that (i) the remaining alkali component thermally diffuses into the recording medium during the fabrication to form a compound together with the components of the recording medium, thereby lowering magnetic properties of the recording medium; and that (ii) the remaining alkali component diffuses with time into the surface of the recording medium and combines with water or a carbon dioxide gas on the surface to form a hydroxide or a carbonate, and this compound becomes a foreign material on the surface, thereby causing surface deficiency or recording jump occur.
In the light of the above, on these days in which the recording density is tremendously increased, the foregoing problems are one of the factors of disturbing the enhancement of recording density. Accordingly, much more than so far, a substrate with less alkali elution, especially a substrate having a low content of a lithium ion with a smaller ion radius which is a main cause of the generation of alkali corrosion is eagerly demanded.
As an application of the substrate for information recording medium, several crystallized glasses are known. For example, an SiO2—Li2O—P2O5 based crystallized glass disclosed in JP-A-2000-302481 has lithium disilicate (Li2O.2SiO2) and α-quartz as a main crystal phase and is a material exhibiting excellent physical properties as the information magnetic recording medium. However, since the amount of an alkali component is relatively high, a problem of the generation of damaging a magnetic film to be caused due to a lithium ion deposited from an end surface of the substrate after fabrication cannot be connived. Also, the surface roughness of the substrate cannot be reduced (<3 angstroms) so that it cannot be adapted with high-density recording.
An Li2O—Al2O3—SiO2 based crystallized glass disclosed in JP-A-2000-184624 has lithium disilicate (Li2O.2SiO2) as a main crystal phase, has a low Li2O content as from 4% by mass to 8% by mass and is a material exhibiting excellent physical properties as the information magnetic recording medium. However, for the purpose of minimizing the deposition of a lithium ion, the content of the Li2O component is merely made low, and therefore, the viscosity at the time of melting the raw glass becomes high. Thus, it is not suitable for press molding.
An Li2O—Al2O3—SiO2 based crystallized glass disclosed in JP-A-2001-48584 is a crystallized glass for optical filter containing lithium disilicate (Li2O.2SiO2) and at least one member selected among α-quartz, an α-quartz solid solution, α-cristobalite and an α-cristobalite solid solution as a main crystal phase. However, it has a high viscosity at the time of melting the raw glass and is hardly press molded into a thin sheet, and therefore, it is not suitable for an application of information recording medium.